Seismic data acquisition systems conventionally use cabled networks comprising electronic units whereto ground movement sensors are connected.
FIG. 1 illustrates schematically a seismic data acquisition system according to a first known solution, based on the use of analog sensors 4.
For the sake of simplification, each reference 4 designates an analog sensor and its corresponding housing and casing (as detailed below with FIG. 6).
To collect the seismic data (geophysical data), one or a plurality of seismic sources (not shown in FIG. 1) in contact with the ground are activated to propagate omni-directional seismic wave trains. The sources may among other things consist of explosives, falling weights, vibrators or air guns in marine environments. The wave trains reflected by the layers of the subsurface are detected by the analog sensors 4, which generate an analog signal characterising the reflection of the waves on the geological interfaces of the subsurface.
The analog sensors 4 are generally referred to using the term “analog geophones”. As shown in FIG. 6, they are generally interconnected in groups of sensors by a two-conductor line 5 (or a three-conductor line for a serial-parallel configuration) to form clusters referred to as “strings of analog geophones”. To this end, each analog geophone is mounted in a mechanical housing (or cartridge) 62. This mechanical housing 62 of analog sensor is inserted with mechanical tolerances inside a casing 61 (made of plastic in general) which shape is dependent of the type of area to investigate (marsh, land . . . ). The two-conductor line 5 is usually moulded to the casing 61.
Each of the strings is connected to an acquisition device 3 (several strings can be connected to the same acquisition device). To this end, the acquisition device is also mounted in a mechanical housing which comprises a connector 63 with two contacts, adapted to cooperate with a connector 64 of the same type placed on the two-conductor line 5 (i.e. the cable of the string).
A string of analog geophones allows to filter the noise (spatial filtering), since the analog information circulating on the two-conductor line 5 (towards the acquisition device 3) is the average of measurements made by each of the analog geophones.
The acquisition devices 3 are generally referred to using the term “Digitizer Unit”. They are interconnected by a cabled network (e.g. a four-conductor line), perform the analog to digital conversion of analog signals coming from the groups of sensors and send the resulting digital seismic data to a central recording system 1 (also referred to using the term “central data processing unit”), via intermediate collection devices 2 (also referred to using the term “concentrator device”). The central recording system 1 is usually onboard a recording truck.
The acquisition devices 3 also performs other functions, notably: synchronisation with the central recording system 1, processing of the seismic signal and interfacing with the digital network (i.e. transferring seismic data to the central recording system 1, receiving and processing commands received from the central recording system 1).
FIG. 2 illustrates schematically a seismic data acquisition system according to a second known solution, based on the use of digital sensors 20. Identical elements are designated by the same numerical reference sign.
The digital sensors 20 are generally referred to using the term “Digital Unit”. Each digital unit includes a sensor which is a micro-machined accelerometer (also referred to using the term “MEMS-based accelerometer”. MEMS being the acronym for “Micro-Electro-Mechanical System”).
Comparing with FIG. 1, each digital unit replaces an acquisition device 3 and the string or strings of analog sensors 4 connected to it via a two-conductor line 5. As the acquisition devices 3 of FIG. 1, the digital units 20 are interconnected by a cabled network (e.g. a four-conductor line) and send the digital seismic data to a central recording system 1, via intermediate collection devices 2.
In a known alternative embodiment, the acquisition device 3 or the digital sensors 20 use a wireless network to communicate with the intermediate collection devices 2 and/or the central recording system 1.
In another known alternative embodiment, the acquisition device 3 or the digital sensors 20 use have a memory sufficient for a later seismic data harvesting.
The digital sensors offer advantages over analog sensors (particularly in terms of bandwidth and sensibility stability). However, the solution of FIG. 2 is not optimal if one wishes to make an acquisition with a high number of digital sensors (for example a thousand digital sensors, and thus a thousand digital units 20) by acquisition line (each acquisition line being connected to an intermediate collection devices 2). Indeed, in this case, all the digital units of an acquisition line are connected in series, which requires a robustness of each digital unit (in quality design and manufacturing, which is expensive).
In addition, the solution of FIG. 2 can not have multiple-sensor strings on a same acquisition line (as opposed to the solution of FIG. 1, with strings of analog geophones). However, there are advantages to use a string of digital geophones, notably:                a higher fidelity and stability of measures; and        a string of analog geophones gives a measure which is the average of the measurements, while a string of N digital geophones gives N measures. The access to these N measures allows for a spatial filter (combination of the N measures) more effective.        
At least for these reasons, and for cost reasons, the inventors have come to the conclusion that it would be interesting to implement strings of digital sensors, using the well-proven string technique comprising a two-conductor line 5 (i.e. the cable of the string) and associated connectors 63, 64, casing 61 and housing 62 (see FIG. 6).
In a particular embodiment, the proposed solution should also allow to connect only one digital sensor to the acquisition device, via a two-conductor line (i.e. a connection according to a bus topology, also referred to as a branch topolgy).
Unfortunately, there is currently no solution as discussed above, combining the concepts of string and digital sensor. In other words, it is not possible today to use the existing cable (two-conductor line 5) of the strings by replacing, in mechanical housing (or cartridge) 62 of analog sensors, these analog sensors by digital sensors.
As explained above, this mechanical housing 62 of analog sensors is inserted with mechanical tolerances inside a casing 61 (made of plastic in general) which shape is dependent of the type of area to investigate (marsh, land . . . ).
It must be noted that it is was not obvious for the Man skilled in the art, at the time the present invention was made, to fins a technical solution allowing to connect one or several digital sensors and an acquisition device together, using a two-conductor line usually used to connect one or several analog sensors, which further is a poor quality cable.
Indeed, currently, a given digital unit is connected to another digital unit, via a two pairs line cable, which is a good qualify (and therefore expensive) cable, by which:                the given digital unit receives electrical power (from mid-point of both pairs), used in particular by the digital sensor (MEMS-based accelerometer) included in the given digital unit;        the two pairs are connected to digital unit and are used in full-duplex mode: one pair as input for commands and synchronization (sampling clock); one pair as output for seismic data obtained by the digital sensor (MEMS-based accelerometer for example) included in the given digital unit (these seismic data being intended ultimately to the central recording system 1);        the given digital unit receives a sampling clock (common to tall digital sensors included in the various digital units), used by the digital sensor (MEMS-based accelerometer) included in the given digital unit. The original sampling clock is a high-precision clock (such as a quartz oscillator) contained in the central recording system 1. The digital units are frequency-dependent by means of a low phase noise analog phase lock loop (PLL) that extracts the sampling clock from the data. Low phase noise is mandatory to achieve accelerometer ultra low noise specifications. To achieve low phase noise with such a PLL, it is mandatory to always have data on the input pair hence to be in fall duplex mode. Indeed, periods with no data fed into PLL would lead to PLL drift and phase noise. That's the reason why using such a PLL imposes the use of two pairs.        the given digital unit sends at least quality control data or seismic data obtained by the digital sensor (MEMS-based accelerometer) included in the given digital unit (these seismic data or quality control data being intended ultimately to the central recording system 1);        the given digital unit receives command data (that the central recording system 1 sends to the given digital unit).        
It is important to note that the Man skilled in the art is faced with the following dilemma: the digital sensor should receive the sampling clock via a poor quality (low cost) standard geophone cable (i.e a two-conductor line). But in the facts, this is impossible. Indeed, the noise on the line does not allow it. More precisely, the phase jitter of the data, and hence of the sampling clock after recovery by the PLL, on this cable would be too high. In other words, attenuation and distortion induced by such a cable degrade the temporal precision of a clock that would be transmitted on that cable. Moreover such a cable gets (“picks up”) the ambient electronic noise that would degrade even more this clock.
The Man skilled in the art is faced with another problem: the use of a phase lock loop (PLL), in order to recover the sampling clock coming from the central recording system 1, induces a size which is not compatible with a desired objective of placing the digital sensor in a housing containing usually an analog sensor.
The use of a PLL also induces a great power consumption which is not compatible with a desired objective to reduce the power consumption of the acquisition device.
Thus, the Man skilled in the art has therefore no incentive to pursue the course of trying to combine the concepts of sensor string and digital sensors.